Crossing Barriers
by FuziPenguin
Summary: Sideswipe and Sunstreaker finally get some alone time after landing on Earth. Someone accidentally stumbles upon them. Slash


**Title:** Crossing Barriers  
**Fandom:** Transformers:Prime with a mix of Bayverse/G1 frame styles, colors, and weapons for the Twins  
**Author:** FuziPenguin  
**Pairing/Characters**: Jack, Sideswipe/Sunstreaker  
**Rating:** NC17 (Sticky, spark-sex)  
**Word Count:** 3805  
**Summary**: Sideswipe and Sunstreaker finally get some alone time after landing on Earth. Someone accidentally stumbles upon them.  
**Warnings:** AU as the twins haven't entered TF:P; as such, there are bits and pieces of fanon/canon everywhere regarding the Twins' background and behaviors. Vaguely a sequel to Down on One Knee. I think this might end up being a series, tentatively titled "Things Jack Sees".  
**Disclaimer:** This is a piece of fiction. No harm was intended in the creation of this work. All rights belong to the original creators.

* * *

"Sunny…" Sideswipe murmurs, arching his back struts to brush his chassis against his brother's.

Sunstreaker tightens his grip on Sideswipe's wrists, a tiny prelude to Sunstreaker slamming the back of Sideswipe's arms against the wall.

"Don't call me that," Sunstreaker growls against Sideswipe's audio, hot vented air washing over the sensors within.

Sideswipe's engine rumbles as he slides a knee up the outside of Sunstreaker's thigh. "Awww, baby… it's just you and me. No one else to hear the pet names, Sunshine."

This time the slam is even more forceful, although careful of Sideswipe's healing injury. "_I'm_ here!" Sunstreaker snaps, denta nipping the edge of an audio horn.

Purring at the surge of pain/pleasure, Sideswipe's leg hooks over Sunstreaker's waist and yanks him forward, eradicating the scant inch between their frames.

"That ya are. And here I am… all pinned and helpless. Whatcha gonna do? You gonna punish me? I do call ya an awful lot of nicknames," Sideswipe says coyly, rocking his pelvis forward to brush his heated panel against Sunstreaker's.

Sideswipe's spark whirls in his chassis, eager and aching. It's been_ so long_ since their last interface. So long living with the sensation of an empty valve and even worse, an incomplete spark.

But they're here now, on Earth, in _Prime's_ unit. They can finally, _finally_, let their guard down for a little while and trust in the skills of the mechs that Prime has gathered around him. And the Autobot leader himself is only floors above them; anything attacking the base would have a hard time getting past _him_.

Sideswipe can practically feel the crackle of overload in his lines; he's starting to shake for it, tiny shivers wracking his frame. He had wanted to make this slow, draw it out, but he hasn't had Sunstreaker in _ages_. It will probably end up fast and hard; but that's kinda their style anyway.

"I'm going to do whatever I want to you," Sunstreaker answers, vocalizer pitched an octave lower than normal.

Sideswipe arches again, hands twisting out of Sunstreaker's grip. Easily; Sunstreaker lets him go, lets him run his hands along Sunstreaker's sides, fingers dipping into transformation seams to pluck at tension wires and support cables. Sunstreaker's shaking too, small tremors that prove he's eager as well.

Sideswipe shoves at Sunstreaker's shoulders, pushing him back far enough so Sideswipe can bend and extend his glossa to lap at the near invisible seam running down the center of Sunstreaker's chest. Sunstreaker's hands spasm against Sideswipe's waist; groaning, he lets Sideswipe swing them around so now Sunstreaker's back rests against the wall.

"I gotta hand it to them; their waxes aren't that bad," Sideswipe murmurs against the widening crack in the armor. The tang of citrus lingers on his glossa from his brother's latest excursion to the car wash.

"Passable," Sunstreaker grunts in agreement. "More!"

"So demanding," Sunstreaker says slyly, grinning up at his brother. He teases at the split in Sunstreaker's chest plating, already feeling the tingle of Sunstreaker's spark energy in his mouth components. Before Sunstreaker can retort, Sideswipe cups his brother's array, fingers slipping in the lubricant welling up at the panel's edges.

"Mmm. You're already so wet," Sideswipe purrs.

He massages the panel with short, firm strokes of his digits. Moments later, the cover snaps aside, and Sideswipe's palm is filled with a rush of liquid warmth. It overflows, droplets of lubricant sliding down Sideswipe's wrist to plop on the floor beneath their feet. Immediately, Sideswipe plunges two fingers deep into Sunstreaker's valve, the walls spiraling down tight in rippling waves around the digits.

"Sides…" Sunstreaker groans, his head thrown back and optics tightly shuttered.

Sideswipe marvels at how responsive Sunstreaker is, how much transfluid is leaking from the tip of his pressurized spike. Sideswipe isn't much better; his own spike is knocking at its housing cover and lubricant slicks his thighs. Primus, he _wants_! He wants it all, every bit of it!

"Where… how?" Sideswipe manages, adding a third finger to Sunstreaker's valve and wrapping his other hand around Sunstreaker's spike. He nuzzles the widening chasm in Sunstreaker's chest plating, feeling the latches of his own chest armor release.

"Get… up here," Sunstreaker chokes out, blue light flickering from the depths of Sunstreaker's chest cavity. He weakly yanks on Sideswipe's shoulders.

Doing the exact opposite that Sunstreaker wants, Sideswipe falls to his knees. He's unable to resist taking the head of Sunstreaker's spike into his mouth and sucking, his glossa rapidly catching the seeping drops of transfluid. He moans around his mouthful, optics shuttering closed at the sweet taste, at the shudder that travels down the strong thigh his hand is gripping.

"Slagger!" Sunstreaker groans, hips undulating. Sideswipe can only hum a reply, his chassis plating sliding aside completely. His core armor shifts; his spark dances wildly behind it, excited to join its other half.

Sideswipe sucks harder, fingers plunging deeper. His twin's close; the motions of his hips are arrhythmic, fighting between the urge to push his spike deeper into Sideswipe's mouth and the desire to grind back on Sideswipe's fingers.

Sunstreaker shudders again, a keen beginning in the back of his vocalizer. His hands are trembling on the back of Sideswipe's helm.

"C'mon, Sunny," Sideswipe coaxes, his lips sliding off Sunstreaker's spike, replaced by his hand. A rough stroke has Sunstreaker's knees buckling, and Sideswipe mouths at the closest hip seam, optics steady on Sunstreaker's faceplates. His favorite part is watching his brother come apart, listening to him call out Sideswipe's name.

"Oh my God!"

Sideswipe stills. That is _not_ the voice he had been expecting to hear.

An instant later, his chest plating slams closed, and his battle programs come fully online. Snarling, he shoots to his feet, blades out.

Instinct wars within him: attack the intruder or stand guard over his brother. Sunstreaker is cursing behind him, sluggishly fighting off the haze of an impending overload. His twin's spark is still exposed, vulnerable, and Sideswipe makes a decision. He pushes off from a standing position and two Earth seconds later he is towering over the small organic that cowers on the floor.

"I'm sorry! I'm sorry! I didn't know anyone was down here! I promise I didn't see anything. I didn't!" it protests, shielding its head with its arms.

Every line of code within Sideswipe is screaming at him to plunge the tip of his blade into the organic's midsection, all the way through to meet the concrete. He wants it nearly as badly as he wanted his brother just moments ago. They had been _so close_… they had actually felt _safe_.

Not that this tiny flesh bag could cause either of them much harm. It's the oldest youngling, _Jack_, his processor slowly supplies. But a youngling still, and weaponless.

"I'm sorry, I'm sorry," Jack continues to mutter, his small frame shaking. Sideswipe lowers the blade a bare inch, and the tip indents the cloth covering the youngling's stomach. Jack freezes, his breath catching.

"Please," he whispers, from the fold of his arms.

Sideswipe knows he's not a good mech. Never has been. Too many coding errors to ever be. He relishes fighting, enjoys his enemy's energon dripping from his blades or his hands. But he's not a monster, no matter what they say about him or Sunny.

Still. The lingering taste of Sunstreaker's transfluid on his glossa is no longer sweet, but bitter with incompletion.

"I should kill ya right now," Sideswipe says through gritted denta.

"I really wish you wouldn't," Jack replies faintly, slowly lowering his arms and revealing his wide eyes.

It's so much like something Sideswipe would say that he snorts, amused despite himself. Some of his rage bleeds away.

"Give me a reason why I shouldn't," Sideswipe says, poking the fleshie's midsection a little harder. Jack makes a strangled sound, but otherwise stays still.

"Uh…"

"Stop playing with him," Sunstreaker scolds, coming up behind them.

Sideswipe shoots a glance over his shoulder to see Sunstreaker has put himself put to rights; chest closed and interface array cover securely in place. Sunstreaker holds himself rigidly, arms crossed defensively across his chassis. A buffing cloth dangles from the fingers of one hand.

Sunstreaker's a private mech. There's a certain vulnerability during interfacing that Sunstreaker hates to have others witness; it's why they rarely take lovers. It's also why they don't merge or 'face each other as much as rumor suggests.

In a war, it's uncommon for anyone to eke out a private moment; mechs used to stumble over them time and time again. Some would avert their optics and continue on their way; it was bad enough to know that someone had glimpsed their exposed sparks. But many would gossip about what they had seen or speculate with other mechs in the unit. A bold few had even tried to join in without invitation, especially once talk had inflated their reputation.

After that, Sunstreaker had withdrawn from intimacy, much to Sideswipe's disappointment. It was only when they were certain they would be left alone for a defined period of time that he would lower his guard. And even then, Sideswipe had had to really work at coaxing even the smallest sounds of pleasure from his brother. Sunstreaker was always afraid someone would see or hear.

For days, Sunstreaker'll be quietly mortified that this interfering little organic had seen him so close to shattering apart. The next time they come together, Sunstreaker will continually look over their shoulders, distancing himself just a little. A bit of Sideswipe's anger resurges.

He can't harm the squishie; Optimus will have their heads, benevolent Prime or no. And he can't reassure his twin, not in front of the human. So he does the next best thing; he plays it off, at least for the moment.

Sideswipe grins. "Ya got a little something there," he says, pointing to a smear of lubricant on Sunstreaker's thigh. Sunstreaker checks himself and angrily wipes away the spot. Then he throws the cloth at Sideswipe.

"So do you," he sneers, pointing at Sideswipe's faceplates. Sideswipe catches the cloth mid air, simultaneously sheathing both of his swords. He wipes his mouth with the back of one hand and sees it liberally smeared with fluids.

He leers down at Jack. "Well, I guess I do," he drawls, wiping the cloth across his faceplates and hands as he gets back on to his wheels.

"These corridors are off limits," Sunstreaker tells the youngling, coming over to stand next to Sideswipe. He's very slightly vibrating, likely awash with the feelings of denied pleasure and embarrassment. "Why are you down here?"

Jack doesn't move, looking up at both of them like a petrorabbit does a cyberwolf. "I… I was studying. I needed a break. No one's ever minded before if I come down here as long as I don't go into any of the vaults."

Sideswipe and Sunstreaker exchange disgruntled glances. The only reason they had taken their play to these deserted hallways was because they were forbidden to the human children; or at least they were supposed to be.

Ratchet had taken them aside early after their arrival. The medic had told them he and Optimus realized their desire to spark merge, but also understood the fundamental _need_ to do so as well. Few medics or commanders had ever realized that allowing them the privacy and safety to join their sparks actually made them better fighters, more in tune with one another. That Ratchet had even known this about spark twins and that Optimus had agreed with the CMO's assessment had certainly raised their opinion of Ratchet and the Prime.

The medic had pointed out the base's abandoned sections; the places that no cameras looked into and no humans were allowed to visit. He had encouraged the twins to use these areas, but had also warned them to make sure the human children never saw their activities. There were many things that they shared with their human allies, but details of interfacing were not one of them.

Well, so much for that.

"No one's ever minded before, because no one's ever caught you. Ratchet said no humans down here," Sunstreaker growls, bending over to stab a finger through the air in Jack's direction.

"All right, all right! I get it," he says, slowly getting to his feet. "Though maybe next time you could put a sock on a door or something."

Sideswipe cocks his head to the side, studying the human. "Sock? On a door?"

Humans always had the oddest sayings! Didn't socks go on their feet, as a pitifully inadequate way to protect the delicate flesh they walked on? Why would a door need a sock?

The skin of the youngling's face abruptly turns a startling red color, and Sideswipe worries for a moment that they need to rush him to Ratchet.

"Yeah… you know," Jack says, gesturing through the air at them without meeting their optics. If the hand waving is supposed to convey something, it fails.

Sideswipe bends at the waist so he can better see the human's face. "No. We don't."

"A sock, or other article of clothing such as a tie, placed on the outside door knob generally indicates that the people behind the door are engaging in sexual intimacy," Sunstreaker supplies, sounding as if he's reading it directly from a source. Probably the internet. Sideswipe should have thought of that. The only nice thing about this planet is that it comes with a manual, if an often confusing and disorganized one.

"Yes! That!" Jack says, pointing at Sunstreaker.

Sideswipe glances around, still confused. "There's no door to this hallway."

"It's just a figure of speech!" Jack says, throwing his hands up in the air. His face is still red, and his breathing and pump rate are elevated still, whether from continued fear or some other emotion, Sideswipe isn't sure.

"I don't even… how could… aren't you brothers?" Jack blurts out, finally looking directly at their faceplates. Now the red has a faint green tinge to it. Sideswipe hadn't realized how colorful these humans got; it was actually pretty interesting.

"Yeah," Sideswipe drawls. He thought Ratchet had been pretty clear with their introductions to the rest of the group, humans included, when he had said, 'These are the brothers Sunstreaker and Sideswipe.' "What about it?"

"And you…" Jack makes another complicated hand gesture in the air.

Were the humans devolving back to nonverbal communication? Or was this youngling's development retarded?

"You two were, you know… doing it… right? …sex?"

"Interfacing? We were trying to," Sunstreaker says pointedly. Right now, he's very much exuding 'go away' vibes, but Jack apparently doesn't sense them.

"But… but you're twins!" Jack protests. Sideswipe looks at his brother again and shrugs. He doesn't get why the organic is so adamant in pointing out the fact.

This time, Sideswipe is the one reaching out to the World Wide Web for an explanation as the youngling seems incapable of providing one in a timely manner. After a few seconds of reading, understanding dawns on him.

"Incest," he says, considering the word and all it means to humans. "Sexual relations between immediate familial units are frowned upon in most societies." He makes a face at Sunstreaker; now he understands where the youngling's reaction is coming from.

"Yes!" Jack exclaims.

Sideswipe considers the youngling again. He has a sneaking suspicion that this is one of those topics they're not supposed to broach with any human, let alone a child. But somehow, letting this youngling think that what Sunny and he had been doing was wrong or shameful is even worse.

"Look," Sideswipe begins, "it's bad for your species because combining too much of the same type of DNA causes genetic defects. So if you were to make a sparkling with your mom, it would probably end up stupid or have three legs or something."

Jack takes a few steps backwards, raising his hands as if he's warding off an attacker. "I'm not having sex with my mom!" he protests.

"Right. I know," Sideswipe says soothingly. "It's just an example. We don't have DNA. So it's not a problem for our species."

Jack's arms slowly fall back to his side. "… Ok."

"And me and Sunny, we're like, practically _required_ to 'face each other," Sideswipe continues. "The halves of our spark have to realign every so often or we'll die." That was an exaggeration, but Jack didn't have to know that.

"Your sparks have sex?" Jack asks, incredulous. "Aren't they in your chest? Because it didn't look like you were anywhere near his spark," he tells Sideswipe and immediately turns red again.

Embarrassment, Sideswipe finally decides. The prankster in him notes that that has potential; but later. Teasing the youngling won't help him understand.

"Spark merging is just one kind of interfacing," Sideswipe informs him. "It's generally reserved for those who are bonded. Most mechs get by just fine with a valve and spike."

"Valve. And spike," Jack says doubtfully. "Like… a…a vagina. And a penis? So which one of you has what?" Jack asks. Then he shakes his head, backing up another step. "Wow. That's not a personal question at all. Sorry."

"All mechs have both," Sunstreaker replies, becoming more and more irritated judging by his stance. "I spike him, or he spikes me. You going to start giving us all the details of your equipment next?"

The redness begins spreading down Jack's face to his neck. "Hey, I'm sorry," he mumbles. "I don't mean to be rude. You guys can pull stuff down from the internet, but we have to ask."

"Well, are ya done asking? Cuz we were kinda in the middle of something here," Sideswipe says. He has the feeling that none of this is really sinking in, but he's not sure how to explain it any better than they have been.

It shouldn't matter that this tiny, insignificant, human _child_ is looking at them like they just slaughtered a litter of newsparked turbopuppies. They've had a lot of mechs look at them with dismissal, disgust and even horror before; for their origins, their battle lust, Sunny's anti-social behavior or even for his own joke playing. But never for something so fundamental to their very being as interfacing one another. It bothers him that he's feeling the random urge to apologize. They have nothing to apologize for!

"No… no I think I'm done asking. My brain can't take any more. Sorry, again. I'll be sure not to come down this hall next time," Jack says, shaking his head and turning.

Sideswipe watches him walk away for a moment. Then he wheels forward, circling around and ending up in front of the organic. The youngling freezes again, watching him warily. Sunstreaker watches him too, optics narrowed like he's pretty sure he's not going to like what his twin does next.

"Listen. We're two different species; we got different values and taboos. This," he said, pointing to himself and then Sunstreaker, "ain't a no-no for our race. We're not hurting anybody. We were sparked to be together… always. I'm not gonna apologize for us; I'm not gonna be sorry for loving Sunny," he says hotly, defensively crossing his arms over his chassis. "So just deal with it."

Jack gazes up at him for a long moment before he takes a breath and speaks. "You're right," he says, nodding decisively.

"We're literally from two different worlds, two separate cultures. If you had been human, the two of you would probably go to jail. But… you're not human. As far as I can see, you're not hurting anybody. And you love each other, so… I guess it doesn't matter.

"But it's still going to take some getting used to," Jack says, planting his hands on his hips. "I'm never, ever, going to be able to erase what I saw earlier," he says, shuddering. "Put aside the twin thing, and the… the hermaphrodite thing… I just saw two _alien robots_ having _sex_. That's _weird_. I need time to process that."

Sideswipe's pretty bad at reading human facial expressions, but he believes Jack's to be earnest.

"Weird? I've seen some of your species' pornography," Sideswipe scoffs, his anxiety beginning to ebb. The youngling isn't outright accepting them, but he seems willing to try. "Why anyone would want to defecate on…"

"La la la la la!" Jack interrupts, sticking a finger in each ear and scrunching his eyes shut. "I can't be held responsible for what others of my species do in their free time!" After peeking out of one eye to be sure Sideswipe isn't telling any more sexual horror stories, he uncorks one and then the other ear.

"Seriously. After meeting you guys, I really shouldn't be surprised by anything anymore; I _will_ deal. But please no more graphic demonstrations!"

"I think we can manage that," Sideswipe says, grinning. "You just be sure to stay out of places that are off limits."

Already beginning to walk around Sideswipe's wheel, Jack nods. "Oh, trust me, I will! Sorry again, guys!"

Sideswipe watches the human until he moves around the corner. Then he turns to look at Sunstreaker.

"Well. I guess that coulda gone worse," Sideswipe offers.

Sunstreaker just stares at him, orbital ridge raised.

"What?" Sideswipe asks, a little taken back by the intensity of his brother's gaze.

"'I'm not gonna be sorry for loving Sunny'?" Sunstreaker quotes. "You've been watching Lifetime again, haven't you?"

Unexpectedly, Sideswipe squirms in embarrassment. They don't ever really talk about love. They never need to, not when they're spark-close. Hearing the word said out loud is kinda weird. Not bad… just… odd.

"What? I'm not sorry," Sideswipe protests. His spark freezes up in his chest with a sudden thought. "Are you?"

Sunstreaker wheels forward and cuffs him upside the helm. "Idiot," he says, and it's both fond and exasperated.

His hand descends to the back of Sideswipe's neck, and he pulls them together, lipplates meeting Sideswipe's. The lingering charge from earlier flares up as Sideswipe melts against Sunstreaker, glossa tangling with his brother's.

Sideswipe makes a pleased sound as Sunstreaker's lip components leave Sideswipe's mouth to trail down the side of his neck, nibbling at the main energon line there.

"You still wanna…?" Sideswipe ventures. Normally, Sunstreaker'd be backing off by now, too self-conscious by someone catching them to continue.

Drawing back a little bit, Sunstreaker meets Sideswipe's optics directly. "I want you to frag me through the wall."

Every circuit lights up with eager anticipation, and Sideswipe shudders with desire at the words. "I think I can do that," he murmurs.

"Kind of counting on it," Sunstreaker replies with a smirk.

He begins moving backwards, optics lit up in invitation. And all Sideswipe can do is move with him, as he always has, and always will.


End file.
